It is known in the art to use vibration measurement techniques to determine the mass flow of a gas or a liquid. When a tube through which fluid material flows is made to oscillate in a direction perpendicular to that of the flow, Coriolis forces will act on the tube. Measurement of the tube displacement due to these forces then allows for the mass flow to be quantified. A mass flow meter based on this Coriolis principle is disclosed for example in WO 89/04463. The apparatus described therein comprises a throughflow device, such as a tube or a pipe, an exciter device and a plurality of sensor elements, arranged at different positions on the throughflow device. When there is fluid flow through the apparatus, the exciter device is used to oscillate the throughflow device and the oscillations are monitored with the sensor elements. The Coriolis forces occurring due to the material flow cause a phase shift between the oscillations at different locations on the throughflow device. The magnitude of this phase shift is determined using two sensor elements placed at different positions and provides a direct measure of the mass flow through the device. In addition, such mass flow meters may also be used to quantify the density of a fluid in flow, since the resonance frequency of the throughflow device is dependent on the mass of the flowing material. However, the above described method is limited to gaseous or liquid materials and does not offer any information on material texture.
In Applied Rheology 17, 25111-1-25111-7 (2007), the method of laser Doppler forced vibrology applied to soft agricultural products is described. In this measurement technique, a solid food product, such as a melon, is placed on a vibration generator, e.g., a vibrating table, and thus caused to oscillate. The oscillations of the product induced in this way are investigated by measuring its surface motion with a laser Doppler vibrometer. Analysis of the resonance peak frequencies yields the modulus of elasticity of the product which may be helpful to evaluate the product quality, maturity or firmness.
In Measurement Science and Technology 14 (2003) 451-462, a rheological measurement set-up comprising a tube section with a pair of magnets attached to it and a pair of coils fitted on a supporting frame is used to investigate the flow properties of liquids. The tube section containing the flowing liquid is electromagnetically driven to torsionally oscillate, using the magnet/coil pair as an actuator and the oscillatory response of the tube is subsequently picked up by the same magnet/coil pair. The phase shift between the driving signal and the response signal is measured and used to determine the resonant frequency and the damping of the liquid sample. Subsequently, the complex viscosity of the liquid is determined from these latter two parameters with the help of a theoretical model.